a) Field of the Invention
The invention is directed to an aluminoborosilicate glass containing alkali metals and at least a partial volume of silver ions that are introduced into a base glass of silicon oxide, boron oxide, aluminum oxide and at least one alkali metal oxide by ion exchange with alkali metal ions, particularly for gradient index lenses (GRIN lenses), and to a method for the production of GRIN lenses and other optical elements with an at least partially increased or reduced index of refraction.
b) Description of the Related Art
Numerous solutions are known from the prior art for producing gradient index lenses (GRIN lenses). The most effective solutions are based on borosilicate glasses which are suitable for ion exchange in order to increase or partially change the index of refraction.
For example, U.S. Pat. No. 4,902,330 describes a method for the production of GRIN lenses which is based on a two-step process of ion exchange. In the first step, an exchange of alkali metal ions is caused by introducing the starting glass into a melt with silver ions, thallium ions or lithium ions in order to increase the index of refraction uniformly. In the second phase, the introduction of silver ions in the glass from the first phase is partially cancelled through immersion in molten salt with sodium ions or potassium ions in order to generate a gradient of the index of refraction. This two-step process is described for phosphate silicate glasses and borosilicate glasses. For borosilicate glasses, an unwanted coloration by silver colloids is mentioned as disadvantageous.
A fundamental principle for generating differences in the index of refraction in glasses is described in DD 269 615 B5 which discloses glass compositions which are distinguished in that large differences in the index of refraction are achieved in that oxides of trivalent metals (M′″2O3, where M′″=B, Al, Ga) and oxides of monovalent metals (M′2O, where M′=Li, K, Na, Rb) which are contained in the glass have a molar ratio of ≧1.0 and a glass of this kind is brought into contact with silver, silver alloys, molten salt or solutions at temperature between 210° C. and 450° C. The coloring occurring in this process is described as minor. However, the tendency towards crystallization (nepheline) which occurs as a result of thermal shaping processes (rod drawing or float glass process) and which is noticeable as punctiform or chain defects in the GRIN lenses and generates imaging defects or scattered light is disadvantageous.
The teaching of U.S. Pat. No. 6,511,932 B2 is based on similar glass compositions, wherein glass compositions expanded by an obligatory addition of magnesium oxide (of up to 18 mole %) for the ion exchange between silver and alkali is claimed with the advantage of a reduced melt temperature. However, the addition of MgO worsens transmission and contributes to an increased tendency toward crystallization.
In U.S. Pat. No. 5,007,948, which relates to substantially colorless glasses containing silver through ion exchange, the glass has an atomic structure in which the proportion of non-bridging oxygen atoms is less than 0.03 for purposes of preventing coloration by the silver. It cannot be determined whether or not the tendency toward crystallization is also sufficiently reduced.
Further, for the production of GRIN lenses WO 02/14233 A1 describes alkali-free borosilicate glasses with a high silver content in which the high silver content (of at least two cation percent) is present in the base glass by means of block melt without the addition of alkali ions. The gradient of the index of refraction is generated by subsequent ion exchange in alkali-containing molten salt. It is not mentioned in, nor can it be determined from, this reference whether or not visible defects such as those caused by tendency toward crystallization in the glass, and so on, are sufficiently rare or can be excluded.
All of the aforementioned references with ion exchange between silver and alkali have the common drawback that point defects which interfere with imaging systems and illumination systems occur in the finished GRIN lenses when the base glass has a significant tendency to form crystallites or has increased growth as soon as thermal treatment processes (such as thermal shaping processes, e.g., rod drawing or ion exchange processes) are applied to the glass blanks that are solidified from the glass melt.